1. Field of the Invention
The present invention relates to a gold alloy wire, particularly for forming a gold alloy bump in wireless bonding of IC chips, and a method for forming such a gold alloy bump.
2. Description of the Related Art
A wire bonding method is used to connect electrode pads of an IC chip with outer terminals and the like through wires.
A semiconductor device obtained by the wire bonding method is shown in FIG. 5, in which 11 denotes a semiconductor element, 12 an electrode, 13 a gold alloy wire, 14 a lead frame, and H a height of the loop from the top surface of the semiconductor element (hereinafter referred to as "loop height"). Generally, the wire 13 is first bonded to the electrode 12 of the semiconductor element 11, is then arranged into the loop shape, and bonded to the lead frame 14 by ultrasound bonding.
The wire bonding method requires a substantial loop height which makes it difficult to reduce the height of a semiconductor device.
It is therefore desired that the loop height is reduced in the wire bonding method.
Secondly, to remove the above defect in the wire bonding method, the wireless bonding method is known in which outer terminals or electrodes of a circuit board are bonded to bumps formed on the electrode pads of an IC chip. In this wireless bonding method, the height for the connection can be reduced in comparison with the wire bonding method since the electrode pads are directly bonded to the outer terminals or the like with the bumps therebetween. Thus, the wireless bonding method is advantageous in application to thinner packages.
The methods for forming bumps on electrode pads are disclosed in Japanese Unexamined Patent Publication Nos. 48-7678 and 59-208751 and others. These methods comprise heating an end of a wire to form a fused ball, pressing and bonding the fused ball to the electrode pad of the IC chip by means of a pressing jig, pulling the pressing jig with the wire upward to break the wire at a portion of the wire close to the ball, by which a bump is formed on the electrode pads. These methods allow use of an apparatus for the conventional wire bonding method without any change and are excellent in productivity, but are disadvantageous in that the length of the wire left on the ball, so-called tail length, cannot be uniformly short. If the tail length is long, at the connection step, the tail on the bump is bent to protrude transversely, so that a short circuit may be caused.
To solve the above problem, Japanese Unexamined Patent Publication No. 62-152143 discloses a method in which after the ball is pressed and bonded to the electrode pad, a notch is provided to the wire at a location close to the ball by means of a cutting jig, after which the wire is pulled to stably obtain a bump with a short tail. This method allows to stably obtain a bump with a short tail but an additional step is required and the productivity is low.
The preferable material for forming a bump is gold and the preferable material for forming an electrode pad is aluminum. It is, however, known that when the ball formed by heating an end of the gold wire is pressed and bonded to the aluminum electrode pad, an Au--Al intermetal alloy is formed at the connection portion between the gold ball and the aluminum pad. When the connecting portion is held at a high temperature, the amount of the Au--Al intermetal alloy increases and the bond strength at the connecting portion decreases. Therefore, provision of an improved bump material for bonding electrode pads with outer terminals, etc. is demanded.
An object of the present invention is to provide a gold alloy wire which allows a high bond strength even after a bump on an electrode pad is exposed to a high temperature for a long time (hereinafter called a "high temperature bond strength"), and a method for making a bump using such a wire.
Another object of the invention is to provide a gold alloy wire which stably provides a short tail when the wire is used in a method in which a fused ball is formed at an end of the wire, the ball is bonded onto an electrode pad of an IC chip, and the wire is pulled upward to break without a notch on the wire (hereinafter called a "simple pull-derived tail").
A further object of the invention is to provide a gold alloy wire which allows reduction of the loop height when wire bonding is conducted.